Mysterious sea creature eats bluebottles, floats upside down

Cassie Chloe posted this photo on the Ballina Information Exchange Facebook page, asking if the creature was dangerous.

"Saw heaps of these washed up on Angels Beach ... my friend said they are Glaucus Atlanticus (blue dragon/sea slug)," she posted.

According to the Australian Museum, the blue sea slug is "beautifully adapted for life floating upside down in the sea".

They only grow to about 4cm, and look very different in and out of the water.

Glaucus atlanticus, or blue dragon, in the water.

"Blue sea slugs feed almost exclusively on the tentacles of bluebottles," the Australian Museum website explains.

"Interestingly, the nematocysts (stinging cells) on these tentacles pass through the blue sea slug intact.

"The slug can then use these stinging cells in its own defence."

In fact, the blue dragon can select the most poisonous stinging cells from the bluebottle and store them for later use. If there are no bluebottles around to feed on, the slugs become cannibals.

They usually wash onto our beaches after strong onshore winds, and are often accompanied by an influx of bluebottles.

Last year, when hundreds of the blue dragons washed up on a Yamba beach, Southern Cross University professor Steve Smith said they were "some of the most amazing looking marine creatures you will ever see".

"Even though they're beautiful they can actually sting you, especially with the wings," he said.

"They actually swallow a bubble of air so they float, which makes them more susceptible to washing up on the beach."

Prof Smith said localised strandings of the blue dragon sea slug happened on the east coast every four to five years.